1965 JTM45 with EL34s: Dropping Resistors?

[Trouserpress]

I'm looking at a picture of a most original JTM45 that's running on EL34s - but those aren't really a good match for the old RS transformers, right?. Between V4 and the board you can see 3 grey components (most likely resistors?) that might probably have been put in to take some high voltage stress off of the power tubes.

I'm no engineer and not quite sure if that would be a proper arrangement. Please take a look and tell me what you think. Thanks,
Frank

 

[Creach]

I'm looking at a picture of a most original JTM45 that's running on EL34s - but those aren't really a good match for the old RS transformers, right?. Between V4 and the board you can see 3 grey components (most likely resistors?) that might probably have been put in to take some high voltage stress off of the power tubes.
View attachment 158666
I'm no engineer and not quite sure if that would be a proper arrangement. Please take a look and tell me what you think. Thanks,
Frank

Hello Frank,
It should be a 1K resistor going to V4, and from this 1K a 470R on V4 and a 470R on V5.
On your picture the 1K resistor was replaced by the grey resistors and the 2 x 470R was replaced by a wire.
Here is a schematic and a picture of a JTM45 (with kt66) from 1965 that i saw a few years ago.

 

[Trouserpress]

Thanks Creach, so the original 1k(2w) in series with 2x 470R(1w) have been replaced by 3 assumably sturdier grey resistors and 2 leads. I wonder what value these might be and if that’s really all you have to do to complete a KT66 to EL34 conversion….???
I would have to buy it to get some more information (I‘m interested in doing so).

 

[Creach]

Thanks Creach, so the original 1k(2w) in series with 2x 470R(1w) have been replaced by 3 assumably sturdier grey resistors and 2 leads. I wonder what value these might be and if that’s really all you have to do to complete a KT66 to EL34 conversion….???
I would have to buy it to get some more information (I‘m interested in doing so).

Right after the JTM45 is the JTM50 with el34.
The difference is JTM 50 has no resistor at all.
But for el34 a 1K can be placed instead of the 470R (and no need to have the first commun 1K).

 

[Helmholtz]

I'm looking at a picture of a most original JTM45 that's running on EL34s - but those aren't really a good match for the old RS transformers, right?.
View attachment 158666

Judging from the OT wires this amp has a Drake OT.
Either a 784-103 (8k) for KT66s or a 784-128 (3.5k) for EL34s.
If intended for EL34s the bias supply should have a 220k series resistor.

 

[Trouserpress]

Here's a foto from above. Does the shape tell what kind it is?

 

[Helmholtz]

Sorry, no.
Drake 103 and 128 have the same shape.
But it's obviously a Drake.
What are the secondary wire colors?

 

[2L man]

If options are those two the OT Winding Ratio/impedance is very easy to measure in few minutes. Remove power tubes and rectifier tube and there does not come HV to secondary circuit***. Then bend two wires to U and push between 2 and 3 on other power tube socket and between 7 and 3 on another. Then turn mains switch On and there come 50Hz or 60Hz sound from loudspeaker when filament AC go to OT primary thru Anode sockets. Disconnect loudspeaker and measure output AC voltage and AC between U-wires.

***Keeping HV/stby switch Off make that double safe and removing HV fuse make it triple safe.